This invention is primarily related to process plant control systems and specifically relates to a new and novel monitor for determining at least three different conditions in the circuit of various types of electrical equipment.
In general, annunciators are used for process alarms of the type wherein industrial miniature oiltight pushbuttons and switches are used to start and to stop equipment with various light arrangements being used to indicate the run status of the equipment being controlled. In modern process plants most equipment is run by a large number of electric motors, whose operation is very critical to safe and continuous operation of a process plant.
Commonly the monitoring of motor failures has been implemented in one of three methods. By referring to FIGS. 1-3 of the drawing there is shown three typical monitoring methods of monitoring a simple process having a surge tank 10, one agitator 12, and two pumps comprising one active pump 14 and a spare pump 16. The agitators 12 are driven by electric motors 18, and the pumps 14 and 16 are also driven by separate motors which are not shown in the drawing. Each motor has an associated run light located on the control panel for the process equipment. In the example shown in FIGS. 1-3, the run light for the active pumps 14 would be L1-A, while the run light for the spare pumps would be L1-B. In a similar manner, the agitators 12 would have a run light L1-C. The tanks 10 have a continuous incoming product flow through the pipes 20 which is being pumped out through the pipes 22. A level indicator LI-1 is provided as well as a high/low level alarm LAHL-1 which is mounted in the control room.
The following discussion is related to FIGS. 1-3 for simplicity and ease of understanding the merits of the invention, but the basic invention is not to be limited in scope to only the illustrated process and could, for example, be used on many other processes such as a distillation column having as many as six or more pumps.
Commonly motor failures in process plants of the type illustrated in FIGS. 1-3 have been implemented in one of the following three known methods:
1. NO FAILURE ALARM (FIG. 1):
In this example the motors 18 as well as the motors for the pumps 14 and 16 are not connected to any alarm system. An operator is not immediately alerted to any particular motor failure, but will realize a failure by the consequential results in the process some time later if he is alert. In the example, should the motor for the active pump 14 fail, the level in the tank 10 will begin to rise, and there will be a loss of product downstream in the pipe 22 causing a secondary upset in the process. The operator will probably be alerted by the secondary upset caused by loss of product flow downstream or by a high tank level (LAHL-1) when the tank 10 has filled up. If the operator had been alerted to an immediate failure of the active pump 14, he could have switched to the spare pump 16 preventing the secondary upset. Should the agitator 12 fail, a secondary result may be a plugged active pump 14, plugged piping, and poor product downstream in the pipe 22. In either case, the operator is not alerted to the primary failure at the time, but only to the consequential results later. This could result in loss of product, off-grade product, hazardous situation, or a complete plant shut-down due to the failure of one motor.
2. COMMON MOTOR FAILURE ALARM (FIG. 2):
In this example auxiliary contacts from a random number or specific groups of motor starters for the active pumps 14, spare pumps 16, and agitator 12 are connected together to a common annunciator UAL-2 input. Any motor within the group that is stopped normally or fails will cause an alarm. When a failure has occurred in this manner, the operator must scan the control panel to determine from the motor lights on the panel or other secondary indications which motor within the group has failed. The annunciator window on the panel would be continuously illuminated due to the spare pumps in the system. This method of connection saves cost and panel space, but does not direct the operator to the particular problem. On the average, approximately 60% of the motors will be running during normal plant operation resulting in much time being lost in determining which motor in the process has failed.
3. ALARM EACH MOTOR (FIG. 3):
In this example a set of motor starter auxiliary contacts are connected into an individual annunciator input XAL-4 on the spare pump 16; XAL-5 on the active pump 14; and XAL-3 on the agitator motor 18. The annunciator on the panel indicates only that a motor has stopped and does not distinguish between a motor that has failed and one that has been normally stopped by the operator or by the process control. This arrangement has the following disadvantages:
A. One motor utilizes one annunciator window on the panel, increasing annunciator size and panel size, and wasting valuable and expensive face of panel space.
B. Most process pumped systems utilize two pumps (one active and one spare). Therefore, spare pumps and other motors normally not in use would have an illuminated annunciator window on the panel. An illuminated annunciator window on the panel in the general context implies there is a problem that has been operator-acknowledged, but not corrected. This is not the case for spare pumps.
C. The operator gets an alarm when a motor is stopped intentionally by the operator or by the process control.
D. A lengthy and expensive cable is required from the motor control center to the control room for each motor to be alarmed.
E. The visual display is in the annunciator window matrix on the panel which is physically remote from the motor control pushbuttons and indicator lights.
The previous discussion describes motor failure alarms in plants utilizing conventional switches, pushbuttons, and lights for operator interface and the conventional hard-wired control schemes for controlling the motor. FIGS. 4-6 indicate schematically the three most common motor control schemes employed in prior art process control. The primary difference in the three schemes is in the status indication lights L1, L2, and L3. The status lights L1, L2, and L3 may be a separate unit or included in the pushbutton or switch assembly 36 or 38. The contacts for stop 38 and start 36 may be individual pushbuttons, individual switches, or a single unit switch to perform both functions. Some users alter the arrangement to a small degree, but basically the function is the same. A contact closure 36 or 42 closes the motor starter, and a seal in contact 40 maintains the starter coil energized until a contact 44 or 38 (stop contact) breaks the circuit stopping the motor.
Other monitoring equipment has been marketed for controlling motor operation and simultaneously providing status and failure indication. These prior art designs utilize special circuits for controlling the motor with design concepts including complicated and expensive multiplexing of the control signals, computer control, microprocessor control, or including a combination annunciator and stop/start station in one enclosure. Such designs have not gained wide acceptance because the users are slow to relinquish the proven reliability of the conventional control schemes and hardware. Another drawback in these designs is the use of such highly specialized circuits requires a decision making by plant management at the early stages of plant design, and their adaptation of the complicated circuitry to an existing plant often requires major costly changes which many companies are reluctant to make. Another problem associated with the prior art alarm schemes is the lack of any motor status memory being available during a power outage. After the power is restored, the operator must identify which motors were running prior to the outage and which units are blocked out as spares. This can require a substantial amount of time and eliminates the possibility of a quick re-start after a momentary power dip.
A prior art search of the applicants invention in the U.S. Patent Office disclosed patents in the following two very general categories:
A. Multiplex Transmission.
These systems are used to provide multi-information data over the same transmission line and are not considered pertinent to the applicants invention.
B. Two State Indicating Device.
These systems were an improved version of an annunciator and a specialized monitoring and display system capable of visual display and/or alarming the condition of a two state device. Such systems provide the appropriate outputs when a contact or variable is determined to be in an off normal condition. The following brief discussion of each patent uncovered in the prior art search is given for purposes of explanation of that particular prior art:
U.S. Pat. No. 3,714,646
This device is an annunciator system for multiplexing the monitoring at a central location with the operation of the equipment being located at remote points. The system is designed to detect and identify more than one alarm condition in a group of alarm points without having to first correct the initial alarm condition detected. The circuit utilizes diode-transistor logic.
U.S. Pat. No. 3,525,988
This is an improved annunciator with a first out feature utilizing an electro-luminescent window instead of an incandescent lamp for the visual alert. The circuit uses SCR and transistor logic.
U.S. Pat. No. 3,688,294
This circuit is an improved annunciator with a first out feature. This circuit utilizes a transistor for the lamp driver and a SCR memory circuit.
U.S. Pat. No. 3,729,734
This circuit is an improved annunciator with a first out feature and a method of displaying annunciator failure or proper operation. This circuit utilizes relay logic.
U.S. Pat. No. 4,023,077
This circuit is a type of control logic device to produce an output signal when at least two inputs signals are in a chosen condition. The circuit was designed to prevent an operator from opening or closing a lathe chuck when the lathe speed is above a chosen speed. This is a type of speed interlock device.
U.S. Pat. No. 3,813,655
A central station monitors the condition of a group of remote sensors each of which is connected in a reporter unit which generates a characteristic frequency signal, the modulation condition of which is changed when the sensor goes into the alarm condition. This system is a type of frequency multiplexing.
U.S. Pat. No. 3,805,242
This system relates to the multiplex transmission of data between a centrally or conveniently located computer and at least one process to be controlled having a plurality of sensor signals transmitting end stations and control signal receiving end stations. This circuit is a type of frequency multiplexing.
U.S. Pat. No. 3,939,456
This system was designed to display (steady on light) the status of two state (on-off) devices within a furnace that can cause a furnace to shut down. The status lights are displayed in a physical order that will indicate to a service-man which limit has caused the shut down.
U.S. Pat. No. 3,011,162
A fault indicating system is shown which is capable of responding when one contact opens within a group of serially connected contacts. The system also displays the status and provides an audible alarm.
None of the before mentioned patents provide a means to distinguish between a contact that is in the off normal condition due to an operator-initiated function or some other normal condition. The devices taught in the patents can only distinguish between a normal and an abnormal state. The applicant's invention overcomes these shortcomings by providing a means of determining the following condition of a two-state contact:
1. Normal; PA1 2. Off-normal due to an abnormal condition; PA1 3. Off-normal due to a normal condition. PA1 1. Alert an operator to an equipment failure; PA1 2. Alert an operator to an electric motor failure; PA1 3. Provide memory of motor- or equipment-run status during a power outage; PA1 4. Alert an operator of a failure to start or to complete a commanded operation during a prescribed time period. PA1 1. Pulse to stop, immediate indication that equipment has stopped; PA1 2. Sustained pulse to stop, immediate indication that equipment has stopped; PA1 3. Pulse to stop, delayed indication that equipment has stopped; PA1 4. Sustained pulse to stop, delayed indication that equipment has stopped. PA1 A. zero speed switch; PA1 B. current switch indicating loss of current to equipment; PA1 C. voltage switch indicating loss of voltage to equipment; PA1 D. pressure switch; PA1 E. torque switch; PA1 F. flow switch. PA1 A. higher contact interrogation voltages can be used. Standard available is 12 VDC without photo isolation. PA1 B. the photo isolation allows interrogation of contacts with AC or DC ranging from 24 volts to 130 volts. PA1 C. the photo isolation allows interrogation of a contact already in use without loading either circuit. PA1 D. high input/output isolation is provided. PA1 E. the photo isolation allows interrogation of the run status light for the run/stop status.